Method and apparatus to limit output power in a switching power supply

ABSTRACT

Techniques are disclosed to adjust a current limit in a switching regulator. One example switching regulator includes a switch to be coupled to an energy transfer element of a power supply. A controller to generate a drive signal is coupled to control switching of the switch to regulate an output of the power supply. A current limiter is included in the controller to adjust the drive signal to limit a current though the switch to a variable current limit value. The variable current limit value is set by the current limiter in response to an input line voltage to be coupled to the energy transfer element.

BACKGROUND

1. Technical Field

The present invention relates generally to electronic circuits, and more specifically, the invention relates to switched mode power supplies.

2. Background Information

A typical requirement for power supplies of electronic equipment is that they limit their output power. One reason to limit output power is to meet the requirements of safety agencies for prevention of personal injury. Another reason to limit output power is to avoid damage to electronic components from an overload.

Power supplies typically have self-protection circuits that respond when an output becomes unregulated for a specified time. However, if output power is not limited, a fault at a load can consume enough power to cause damage or to exceed regulatory requirements while the outputs remain regulated. Thus, the self-protection feature can be ineffective if the power supply can deliver too much power.

A common way to limit output power of a switching power supply is to limit the current in a power switch at the input of the power supply. The maximum output power is related to the peak current in the switch. Inherent delays in the responses of electrical circuits create an error between the desired limit for peak current in the switch and the actual maximum peak current in the switch. The error is greater at higher input voltages, causing the maximum output power to be greater at higher input voltages than it is at lower input voltages.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention detailed illustrated by way of example and not limitation in the accompanying Figures.

FIG. 1 is a functional block diagram of one embodiment of a switching power supply that may limit output power in accordance with the teaching of the present invention.

FIG. 2 is a graph of power capability for one embodiment of a switching power supply with respect to the peak current of the switch.

FIG. 3 shows waveforms of the current in the switch for one embodiment of a switching power supply in accordance with the teaching of the present invention.

FIG. 4 shows parameters of timing signals with parameters of the current in a switch of a power supply that may limit output power in accordance with the teaching of the present invention.

FIG. 5 is a flow diagram that illustrates a method to limit output power of a switching power supply in accordance with the teaching of the present invention.

FIG. 6 shows timing signals with waveforms of the current in a switch of a switching power supply to illustrate operation of one embodiment of the present invention.

FIG. 7 is a functional block diagram of one embodiment of the present invention that includes the power switch in an integrated circuit.

DETAILED DESCRIPTION

Embodiments of a power supply regulator that may be utilized in a power supply are disclosed. In the following description, numerous specific details are set forth in order to provide a thorough understanding of the present invention. It will be apparent, however, to one having ordinary skill in the art that the specific detail need not be employed to practice the present invention. Well-known methods related to the implementation have not been described in detail in order to avoid obscuring the present invention.

Reference throughout this specification to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, the appearances of the phrases “for one embodiment” or “in an embodiment” in various places throughout this specification are not necessarily all referring to the same embodiment. Furthermore, the particular features, structures, or characteristics may be combined in any suitable manner in one or more embodiments.

As will be discussed, the power from a switching power supply may be limited according to embodiments of the present invention by limiting the current in a switch of the power supply. For one embodiment, a switch is coupled to an energy transfer element of a power supply with a controller generating a drive signal to control switching of the switch to regulate the output of the power supply. The controller includes a current limiter, which will adjust the drive signal to limit a current though the switch to a variable current limit value. For one embodiment, the current limiter based on the input line voltage of the power supply sets the variable current limit value. For example, the variable current limit may be to a nominal current limit value for nominal or a low input line voltage. If, however, the input line voltage is relatively high, then the variable current limit is set to a reduced current limit value in accordance with the teachings of the present invention. For one embodiment, the controller deduces the magnitude of the input voltage by measuring how long the current takes to go between two values and the variable current limit is then adjusted accordingly.

The variable current limit value for the switch is adjusted according to the input voltage of the power supply to compensate for a delay between the time when the current reaches the current limit and the time when the switch turns off. A lower value of current limit for higher input voltages prevents excess output power at high input voltage. For one embodiment, input voltage may be determined indirectly from a measurement of time to reach current limit from an initial value of zero current when the power supply operates in discontinuous conduction mode. In general, a design can deliver a required output power and also limit the maximum output power over the operating range of input voltage by compensating for the error between desired maximum peak current in the switch and the actual maximum peak current in the switch.

As will be discussed, a measurement of time is used to determine an appropriate adjustment of the desired maximum peak current in the switch to meet the requirements of the design. A current limit threshold for a switch is adjusted in response to a measurement of time during the conduction of the switch to compensate for the undesirable influence of input line voltage on the actual peak current in the switch. For instance, a relatively high input line voltage is indicated for an embodiment of the present invention if an over current condition is identified during a first switching cycle after a skipped switching cycle of the switch.

As will be discussed, it is likely that the power supply will operate in a discontinuous conduction mode of operation in the first switching cycle after a skipped switching cycle. In this situation, the energy in the energy transfer element typically goes to zero before the switch turns on in the next switching cycle. Therefore, if an over current condition occurs during this first switching cycle with the energy in the energy transfer element initially at zero at the beginning of the switching cycle, a high input line condition is indicated, and the variable current limit is set accordingly to the reduced value in accordance with the teachings of the present invention. If, on the other hand, an over current condition is not identified in the first switching cycle after a skipped switching cycle of the switch, then it is assumed that the input line voltage of the power supply is nominal or relatively low, and the variable current limit is set accordingly to the nominal value in accordance with the teachings of the present invention.

To illustrate, FIG. 1 shows a functional block diagram of a power supply that may include an embodiment of a method that limits peak switch current in accordance with the teachings of the present invention. The topology of the power supply illustrated in FIG. 1 is known as a flyback regulator. It is appreciated that there are many topologies and configurations of switching regulators, and that the flyback topology shown in FIG. 1 is provided to illustrate the principles of an embodiment of the present invention that may apply also to other types of topologies in accordance with the teachings of the present invention.

As illustrated in the power supply example of FIG. 1, an energy transfer element T1 125 is coupled between an unregulated input voltage V_(IN) 105 and a load 165 at an output of the power supply. A switch SI 120 is coupled to the primary winding 175 at an input of energy transfer element 125 to regulate the transfer of energy from the unregulated input voltage V_(IN) 105 to the load 165 at the output of the power supply. A controller 145 is coupled to generate a drive signal 157 that is coupled to be received by the switch SI 120 to control switching of switch S1 120. In the example of FIG. 1, the energy transfer element T1 125 is illustrated as a transformer with two windings. A primary winding 175 has N_(P) turns with an inductance L_(P). A secondary winding has N_(S) turns. In general, the transformer can have more than two windings, with additional windings to provide power to additional loads, to provide bias voltages, or to sense the voltage at a load.

A clamp circuit 110 is coupled to the primary winding 175 of the energy transfer element T1 125 to control the maximum voltage on the switch SI 120. In one embodiment, switch SI 120 is a transistor such as for example a power metal oxide semiconductor field effect transistor (MOSFET). In one embodiment, controller 145 includes integrated circuits and discrete electrical components. The operation of switch S1 120 produces pulsating current in the rectifier D1 130 that is filtered by capacitor C1 135 to produce a substantially constant output voltage V_(O) or a substantially constant output current I_(O) at the load 165.

The output quantity to be regulated is U_(O) 150, that in general could be an output voltage V_(O), an output current I_(O), or a combination of the two. A feedback circuit 160 is coupled to the output quantity U_(O) 150 to produce a feedback signal U_(FB) 155 that is an input to the controller 145. Controller 145 also includes a current sensor coupled to receive current sense 140 that senses a current I_(D) 115 in switch S1 120. Any of the many known ways to measure a switched current, such as for example a current transformer, or for example the voltage across a discrete resistor, or for example the voltage across a transistor when the transistor is conducting, may be used to measure current I_(D) 115. The controller may use current sense signal 140 to regulate the output U_(O) 150 or to prevent damage to the switch S1 120.

FIG. 1 also shows an example waveform for current I_(D) 115. During any switching period T_(S) 190, switch S1 120 may conduct in response to drive signal 157 from controller 145 to regulate the output U_(O) 150. When current I_(D) 115 reaches a current limit value I_(PEAK) 195 after a time t_(ON) 180 from the beginning of the switching period T_(S) 190, switch S1 120 turns off and stays off for a time t_(OFF) 185, which is the remainder of the switching period T_(S) 190. The current waveform shows two fundamental modes of operation. The trapezoidal shape 170 is characteristic of continuous conduction mode (CCM) whereas the triangular shape 175 is characteristic of discontinuous conduction mode (DCM).

FIG. 2 shows how the peak current I_(PEAK) 190 is related to the maximum output power of the power supply in FIG. 1. In DCM, the output power increases as the square of I_(PEAK). In CCM, the output power increases linearly with I_(PEAK). The current limit value I_(PEAK) is used to help limit the output power of the power supply. A difficulty in limiting the current limit value I_(PEAK) is that there is always a delay between the time when the current reaches the limit and the time the switch turns off.

To illustrate, FIG. 3 shows how a delay influences peak current in the switch. In the example illustrated in FIG. 3, I_(PMAX) is the maximum desired value for I_(D). A controller having a current limit threshold I_(LIMIT1) that is the same value of I_(PMAX) takes action to turn off the switch when ID exceeds I_(LIMIT1). The unavoidable time delay t_(d) allows I_(D) to exceed I_(LIMIT1) by an amount ΔI_(DELAY) that depends on the delay t_(d) and on how fast I_(D) is changing after it passes I_(LIMIT1). A current limit I_(LIMIT1) produces a peak current I_(PEAK1) that is greater than the desired I_(PMAX). If the delay t_(d) and the rate of change of I_(D) are known, the current limit can be compensated to a lower value I_(LIMIT2) such that addition of ΔI_(DELAY) will give a peak current I_(PEAK2) that is less than I_(PMAX).

A complication in the use of a lower current limit value to compensate for the delay is that in general ΔI_(DELAY) will be larger at higher input voltages than at lower input voltages because I_(D) increases at a greater rate when the input voltage is high. Therefore, a power supply that uses a single compensated current limit I_(LIMIT2) to limit maximum output power to the desired value at a high input voltage would have less than the desired maximum output power at low input voltage. Indeed, if the circuit to limit the power has only one desired limit for peak current such as I_(PEAK2), a design that meets the requirement for maximum power at high input voltage may be unable to deliver the required power at low input voltage.

For one embodiment, a power supply may use a first compensated current limit I_(LIMIT1) at a low input voltage and a second compensated current limit I_(LIMIT2) at high input voltage to limit the maximum output power to a desired value over a wide range of input voltages in accordance with the teachings of the present invention.

To illustrate, FIG. 4 shows example timing signals that are used with the current I_(D) for one embodiment of the invention to determine whether the current limit will be I_(LIMIT1) or I_(LIMIT2). In particular, FIG. 4 shows two full switching periods, T1 and T2 of switch current I_(D) with timing signals I_(LIM), I_(LIMMAX), and D_(MAX). In FIG. 4, current limit signal I_(LIM) is high whenever I_(D) is greater than the current limit. Signal I_(LIMMAX) is a timing reference that is compared to current signal I_(LIM) to determine whether the current limit will be I_(LIMIT1) or I_(LIMIT2). Signal D_(MAX) sets the maximum on time of the switch. The switch is forced off when D_(MAX) is high.

FIG. 5 is a flow diagram for one embodiment of a power supply controller that uses the timing signals of FIG. 4 in accordance with the teachings of the present invention. The flow starts at Block 505 when the switch is off. Block 510 sets a nominal current limit, which for one embodiment corresponds to I_(LIMIT1) in FIG. 4, and is more suitable for a nominal or low input voltage. Block 515 interprets feedback signal U_(FB) to determine whether the switch should turn on or remain off in the next switching period. If the switch is enabled, then Block 520 directs the switch to turn on in Block 525. If the switch is not enabled, then Block 520 directs the switch to be off in Block 545.

Once the switch is turned on, the state of the current limit signal I_(LIM) is evaluated in Block 535. The on time of the switch is compared to the maximum permissible on time in Block 540. Block 545 turns off the switch immediately if I_(LIM) is high or if the on time exceeds the maximum on time t_(DMAX). After the switch turns off, Block 550 directs the flow depending on whether the mode of operation was CCM or DCM when the switch turned on. The mode is DCM if the energy in the energy transfer element goes to zero before the switch turns on. In one embodiment, a single switching period with the switch disabled is sufficient to reduce the energy to zero. Therefore, in one embodiment, Block 550 has a memory of whether or not the switch was enabled during a previous switching period to determine the mode of operation at the start of the present switching period.

If the mode of operation was not DCM, the controller continues with the interpretation of the feedback signal in Block 515. If the mode of operation was DCM, the flow is diverted to Block 555. Block 555 compares the time to reach current limit against the reference time t_(LIMMAX). Although delays in practical circuits prevent exact measurement of the time t_(LIM) to reach the current limit, it is sufficient to measure a signal that includes the delays for an approximate measurement of t_(LIM). For one embodiment, the sum of t_(LIM) and delay t_(d), which is the on time t_(ON) in FIG. 3, is measured in Block 555 as an approximation to t_(LIM) for comparison against the reference time t_(LIMMAX). When the operation is in DCM, the current can reach current limit in less time than t_(LIMMAX) only if the input voltage is high in accordance with the teachings of the present invention.

If the time to reach current limit is less than t_(LIMMAX), the controller sets a reduced current limit in Block 530. The reduced current limit for a high input voltage corresponds to I_(LIMIT2) in FIG. 4. If the time to reach current limit is not less than the reference time t_(LIMMAX), then the controller sets the nominal current limit in Block 510. The latter condition is also true when the switch turns off before the current reaches current limit, causing the controller to set the nominal current limit in Block 510.

FIG. 6 shows several switching periods that illustrate operation according to the flow diagram of FIG. 5. In Period 1, the switch operates at a high input voltage when the current limit has been set at the nominal value I_(LIMIT1) that is appropriate for a nominal or low input voltage. The surplus energy from the high peak current at the high input voltage causes the controller to disable the switch in Period 2. The controller detects a high input voltage condition from the short time to reach current limit in Period 3, and sets the reduced current limit I_(LIMIT2) in Period 4. The operation continues with the reduced current limit until the controller detects a period of DCM operation where the time to reach current limit is not less than the reference time t_(LIMMAX). In Period n, the switch is disabled and the input voltage is low. The controller has determined that the time to reach current limit in a period of DCM was not less than the reference time t_(LIMMAX). Consequently, the controller sets the current limit to the nominal value I_(LIMIT1) in Period (n+1). The current does not reach current limit in Period (n+1) so the switch is turned off by maximum on time and the current limit remains at I_(LIMIT1). The power supply operates in CCM at low input voltage and current limit I_(LIMIT1) in Period (n+2) and Period (n+3).

FIG. 7 shows one embodiment that includes a power switch 736 in an integrated circuit 700. Power switch 736 is a MOSFET that conducts current between a drain terminal 702 and a source terminal 758. Circuits internal to the integrated circuit are powered from an internal voltage V_(CC) 705 that is referenced to source terminal 758. For one embodiment, drain terminal 702 provides internal voltage V_(CC) 705. Internal voltage V_(CC) may be provided from drain terminal 702 or from a different terminal of the integrated circuit by several techniques that are known to one skilled in the art.

A feedback terminal 754 receives a feedback signal U_(FB). A modulator 752 interprets the feedback signal U_(FB) to set an enable signal 744 high or low. An oscillator 756 provides a clock signal 748 and a D_(MAX) signal 746 to determine respectively the length of a switching period and the maximum on time of the switch 736. Switch 736 may be on while D_(MAX) 746 is low. Switch 736 is off while D_(MAX) 746 is high. AND gate 740 sets latch 738 to turn on switch 736 with drive signal 757 at the beginning of a switching period if the enable signal 744 is high. OR gate 742 resets latch 738 to turn off switch 736 with drive signal 757 if switch current I_(D) 706 exceeds the current limit or if signal D_(MAX) 746 goes high.

Switch current I_(D) 706 is sensed as a voltage V_(ID) that is compared to a current limit voltage V_(ILIMIT) by a comparator 704. Resistor 732 with current sources 728 and 730 generates current limit voltage V_(ILIMIT). Current source 730 is switched on and off by p-channel transistor 724. In one embodiment, current source 730 is one-tenth the value of current source 728. Thus, the current limit voltage V_(ILIMIT) increases by 10 per cent to make a nominal current limit 10 per cent higher than a reduced current limit when current source 730 is switched on.

The drive signal 757 that is output by latch 738 is delayed by leading edge blanking time t_(LEB) delay 734 before being received by AND gate 708. AND gate 708 receives the output of current limit comparator 704 and the output of leading edge blanking time delay 734 to provide an over current signal 760. Leading edge blanking time t_(LEB) delay 734 is long enough to allow switch 736 to discharge stray capacitance on drain terminal 702. Discharge of stray capacitance at drain terminal 702 can produce a high drain current I_(D) 706 that temporarily exceeds the current limit, but is unrelated to the output of the power supply. The leading edge blanking time t_(LEB) delay 734 prevents the over current signal 760 from going high during a time t_(LEB) after switch 736 turns on. Over current signal 760 in FIG. 7 corresponds to signal I_(LIM) in FIG. 4 or FIG. 6.

Flip-flop 750 remembers the state of enable signal 744 at the beginning of the switching period. Flip-flop 750 is clocked at the start of every switching period by the complement of D_(MAX) signal 746 from inverter 720. A change in the state of the clocked enable signal 745 from one switching period to the next switching period is detected by XOR gate 716.

XOR gate 716 with delay 718 at one input receives the clocked enable signal 745 to set latch 714 whenever there is a change in the clocked enable signal 745. Delay 718 is long enough to produce an output that sets latch 714. In one embodiment, delay 718 is ten nanoseconds. Latch 714 is set at the beginning of a switching period whenever there has been a change in the state of the clocked enable signal 745 from the previous switching period.

Latch 726 is allowed to set if enable signal 744 is high at the beginning of the current switching period. Inverter 722 resets latch 726 if enable signal 744 is low at the beginning of the current switching period.

Latch 714 is set to indicate DCM operation in the present switching period. DCM is indicated when the output of latch 714 is high. Latch 726 is set to reduce the current limit.

In the embodiment of FIG. 7, the maximum on time signal D_(MAX) 746 is also the timing reference that is compared to over current signal I_(LIM) 760 to determine whether the current limit will be I_(LIMIT1) or I_(LIMIT2). In the embodiment of FIG. 7, t_(LIMMAX)=t_(DMAX), representing an embodiment where the signals I_(LIMMAX) and D_(MAX) of FIG. 4 are identical. For another embodiment, however, it is appreciated that t_(LIMMAX) does not necessarily have to equal t_(DMAX) in accordance with the teachings of the present invention, such as the example illustrated in FIG. 4. Latch 726 will not be set if there is no over current condition detected or the current limit is not reached during the time when D_(MAX) 746 is low. Thus, current source 730 remains switched on by transistor 724 if the over current condition is not detected in accordance with the teachings of the present invention.

It is appreciated that although FIG. 7 illustrates an integrated circuit 700 for an example of the present invention that employs a switching regulator that may skip switching cycles of power switch 736 in response to enable signal 744, other examples of integrated circuits may also be covered in accordance with the teachings of the present invention. For example, a pulse width modulated (PWM) regulator circuit may also be covered in accordance with the teachings of the present invention. For instance, an example PWM controller deduces the magnitude of the input voltage by measuring how long the current takes to go between two values and then adjusts the variable current limit in accordance with the teachings of the present invention.

In the foregoing detailed description, the methods and apparatuses of the present invention have been described with reference to a specific exemplary embodiment thereof. It will, however, be evident that various modifications and changes may be made thereto without departing from the broader spirit and scope of the present invention. The present specification and figures are accordingly to be regarded as illustrative rather than restrictive. 

1. A switching regulator circuit, comprising: a switch to be coupled to an energy transfer element of a power supply; a controller to generate a drive signal coupled to control switching of the switch to regulate an output of the power supply; a current limiter included in the controller to adjust the drive signal to limit a current through the switch to a variable current limit value, wherein the variable current limit value is set by the current limiter in response to an input line voltage to be coupled to the energy transfer element.
 2. The switching regulator circuit of claim 1 wherein the variable current limit value is set to a first current limit value if there is not an over current condition during a first switching cycle that occurs after a skipped switching cycle of the switch.
 3. The switching regulator circuit of claim 2 wherein the variable current limit value is set to a second current limit value if there is the over current condition during the first switching cycle that occurs after the skipped switching cycle of the switch.
 4. The switching regulator circuit of claim 3 wherein the first current limit value is greater than the second current limit value.
 5. The switching regulator circuit of claim 2 wherein the first current limit value corresponds to a nominal or relatively low input line voltage to be coupled to the energy transfer element.
 6. The switching regulator circuit of claim 3 wherein the second current limit value corresponds to a relatively high input line voltage to be coupled to the energy transfer element.
 7. The switching regulator circuit of claim 2 wherein the first switching cycle that occurs after the skipped switching cycle of the switch corresponds to a discontinuous conduction mode of operation of the power supply.
 8. The switching regulator circuit of claim 2 wherein energy in the energy transfer element goes to zero during the skipped switching cycle of the switch.
 9. The switching regulator circuit of claim 2 wherein the over current condition occurs when the current through the switch exceeds the variable current limit value before a maximum on time of the switch occurs during a switching cycle of the switch.
 10. The switching regulator circuit of claim 1 further comprising an integrated circuit including the controller.
 11. The switching regulator circuit of claim 10 wherein the integrated circuit further includes the switch.
 12. A method, comprising: switching a switch coupled to an energy transfer element to regulate a transfer of energy through the energy transfer element; limiting a current through the switch to a first current limit value if there is a nominal or relatively low input line voltage coupled to the energy transfer element; and limiting the current through the switch to a second current limit value if there is a relatively high input line voltage coupled to the energy transfer element.
 13. The method of claim 12 further comprising identifying a nominal or relatively low input line voltage if there is not an over current condition during a first switching cycle that occurs after a skipped switching cycle of the switch.
 14. The method of claim 13 further comprising identifying a relatively high input line voltage if there is an over current condition during a first switching cycle that occurs after a skipped switching cycle of the switch.
 15. The method of claim 13 further comprising identifying the over current condition if the current through the switch exceeds a variable current limit value, wherein the variable current limit value is one of at least the first or second current limit value.
 16. A power supply, comprising: an energy transfer element coupled between an input line voltage and an output of the power supply; a switch coupled to the input of the energy transfer element to regulate a transfer of energy from the input of the power supply to the output of the power supply; a controller coupled to the switch to control switching of the switch; and a current limiter included in the controller to limit a current though the switch to a variable current limit value, wherein the variable current limit value is set by the current limiter in response to the input line voltage to be coupled to the energy transfer element.
 17. The power supply of claim 16 wherein the variable current limit value is set to a first current limit value if the input line voltage is a nominal or relatively low voltage.
 18. The power supply of claim 17 wherein the input line voltage is the nominal or relatively low voltage if there is not an over current condition during a first switching cycle of a discontinuous conduction mode of operation of the power supply.
 19. The power supply of claim 17 wherein the variable current limit value is set to a second current limit value if the input line voltage is a relatively high voltage.
 20. The power supply of claim 19 wherein the input line voltage is the relatively high voltage if there is an over current condition during a first switching cycle of a discontinuous conduction mode of operation of the power supply.
 21. A switching regulator circuit, comprising: a switch to be coupled to an energy transfer element of a power supply; a controller coupled to the switch to control a switching of the switch to regulate an output, the controller to switch the switch periodically in switching periods; and a current limiter included in the controller and coupled to the switch to turn off the switch when a current in the switch exceeds a variable current limit, the current limiter to adjust the variable current limit in response to a measurement of time between values of the current in the switch.
 22. The switching regulator circuit of claim 21 wherein the current limiter is coupled to measure an amount of time for the current in the switch to rise from a first current value to a second current value within a switching period, wherein the current limiter is coupled to set the variable current limit in response to the measured amount of time.
 23. The switching regulator circuit of claim 21 wherein the current limiter is coupled to set the variable current limit to a first current limit value when the measured amount of time is not less than a reference time, and wherein the current limiter is coupled to set the variable current limit to a second current limit value when the measured amount of time is less than the reference time.
 24. The switching regulator circuit of claim 21 wherein the first current value is substantially zero and the second current value is a current limit value.
 25. The switching regulator circuit of claim 21 wherein the regulator is coupled to operate in a discontinuous conduction mode of operation during the switching period.
 26. The switching regulator circuit of claim 21 wherein the controller and switch are included in an integrated circuit.
 27. A method, comprising: switching a switch coupled to an energy transfer element to regulate a transfer of energy through the energy transfer element; measuring an amount of time for a current in the switch to rise from a first current value to a second current value within a switching period; and adjusting a variable current limit in response to the measured amount of time.
 28. The method of claim 27 further comprising turning off the switch in response to the second current value, wherein the first current value is substantially zero.
 29. The method of claim 27 further comprising determining that the switching regulator is operating in a discontinuous conduction mode of operation.
 30. The method of claim 27 wherein adjusting the variable current limit comprises: setting the variable current limit to a first current limit value when the measured amount of time is not less than a reference time; and setting the variable current limit to a second current limit value when the measured amount of time is less than the reference time. switching a switch coupled to an energy transfer element to regulate a transfer of energy through the energy transfer element; measuring an amount of time for a current in the switch to rise from a first current value to a second current value within a switching period; and adjusting a variable current limit in response to the measured amount of time. 